334
Antibody Molecules, Genetic Engineering of
Although all antibody molecules are con-
structed from the basic unit of two heavy
and two light chains (H
2
L
2
), both IgA and
IgM form polymers (Fig. 2). IgA forms
a dimeric structure in which two H
2
L
2
units are joined by a J-chain [(H
2
L
2
)
2
J],
and IgM forms either a pentameric struc-
ture with ±ve H
2
L
2
units joined by a
J-chain [(H
2
L
2
)
5
J] or a hexameric structure
[(H
2
L
2
)
6
], which does not have the J-chain
(not shown). IgM and IgA heavy-chain
constant regions contain a ‘‘tailpiece’’ of
18 amino acids that contains a cysteine
residue essential for polymerization. The
J-chain is a 15 kDa polypeptide produced
by B-lymphocytes and plasma cells (the
same cells that produce the antibodies)
that promotes polymerization by linking
to the cysteine of the tailpiece (Fig. 2). Dif-
ferences in the isotype of the heavy chain
determine the number of carbohydrate
units and the ability to engage in vari-
ous effector functions such as complement
activation, ADCC, and placental transmis-
sion. Differences in the isotype of the light
chain do not appear to signi±cantly influ-
ence the structure or the effector functions
of the antibody molecule.
2
From Mouse to Human Antibodies
2.1
Murine Monoclonal Antibodies
During the normal immune response, a
wide variety of antibodies is produced.
These antibodies, known as
polyclonal
antibodies
(i.e. they are the product of
many different antibody-producing cells),
include antibodies with different variable
regions as well as antibodies with the
same variable regions associated with
different
constant
regions.
Rarely
do
different individuals mount an identical
immune response. This heterogeneity in
the immune response plus ethical and
safety
concerns
has
made
it
dif±cult
to use polyclonal antibodies for many
applications.
A signi±cant breakthrough was made
when
it
became
possible
to
produce
stable cell lines that synthesize a single
homogeneous antibody (Fig. 3). By fusing
a normal B-cell from the splenocytes of an
immunized animal (initially a mouse or a
rat) with a myeloma cell, it is possible to
generate a ‘‘hybridoma,’’ which possesses
the immortality of the myeloma cell and
secretes the antibody characteristic of the
normal B-cell. Antibodies produced by
hybridomas are monoclonal (i.e. they are
the product of a single antibody-producing
cell) and therefore have a single variable
region associated with only one constant
region. The immortality of the hybridoma
ensures the continued availability of a well-
characterized antibody. Once a hybridoma
cell line is developed, it can be grown
in
vitro
or
in vivo
for large-scale production
of the monoclonal antibody, or it can
be used to clone the variable regions
of the monoclonal antibody for genetic
engineering purposes.
Owing to their high af±nity and exquisite
speci±city, murine monoclonal antibodies
seemed to be the ideal ‘‘magic bullets’’ for
diagnosis or therapy of multiple diseases
including cancer. However, the progres-
sion of ‘‘magic bullets’’ from dream to real-
ity has been slow because mouse (murine)
monoclonal antibodies are not the ideal
agents to be administered into a human.
Murine monoclonal antibodies compared
to human antibodies require more fre-
quent dosing to maintain a therapeutic
level of monoclonal antibodies because of
a shorter circulating half-life in humans
than human antibodies. In addition, the
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